1. Field of the Invention
The present invention relates to a radio mobile communication system, and more particularly, to a method of controlling a data rate in transmitting data on reverse link traffic channel in a radio mobile communication system.
2. Discussion of the Related Art
IS-2000 Revision D provides for a mobile communication system enabling packet data transmission to a base station on a reverse-link traffic channel, namely, a reverse packet data channel or R-PDCH, but a mobile station cannot autonomously determine a data transmission rate of a reverse link channel. Serious interference among the mobile stations of a system results should one of the mobile stations be permitted to operate at a data rate much higher than that of others, and the entire system may become unstable. Therefore, reverse link data rates are controlled by a base station, which controls the reverse link data rate of each mobile station by considering a variety of parameters, such as the respective statuses of all the mobile stations and that of a reverse link channel used by the mobile stations, and respectively transmitting to each mobile station information indicative of an appropriate data rate for use by the corresponding mobile station. Thereafter, each mobile station determines its own data rate, based on the information received from the base station, to transmit data on the reverse traffic channel at the determined rate.
The base station may control the reverse link data rate of a specific mobile station by either a general rate control method or a scheduling method. In the general rate control method, a current data rate is set by incrementing or decrementing a previous data rate, thereby enabling control of a reverse link data rate using a relatively few bits of information. In the scheduling method, on the other hand, the base station at all times controls the current reverse link data rate independently, i.e., irrespective of a previous data rate, which requires a great number of information bits. As a preferred method of controlling the data rate of a reverse link channel, the Third Generation Partnership Project 2 (3GPP2) currently adopts a control algorithm, which combines the general rate control method and the scheduling method, assigning a forward grant channel (F-GCH) and a forward rate control channel (F-RCCH) to a specific mobile station via call negotiation with the base station. Here, the mobile station informs the base station of its current status, i.e., its buffer size, its transmission power status, its quality-of-service (QoS) class, and the like, via a reverse-request channel (R-REQCH), so that the base station may transmits a grant message to the mobile station via the F-GCH channel or a rate control bit (RCB) via the F-RCCH channel. Accordingly, the data rate of a reverse-link data transmission, made by a mobile station via the R-PDCH channel, is controlled.
The above methods, i.e., the general rate control method and scheduling method, are explained with reference to FIGS. 1 and 2, respectively. The data transmission rate of a reverse link (traffic) channel, i.e., the R-PDCH channel, is controlled for a specific mobile station by the base station transmitting rate control information in response to the mobile station's status information sent via an R-REQCH channel. The rate control information, transmitted in response to a request from the mobile station, is an RCB transmitted via an F-RCCH channel in FIG. 1 or is a grant message transmitted via an F-GCH channel in FIG. 2. The RCB is essentially a command for a rate hold, a rate increase, or a rate decrease. The grant message includes a maximum traffic-to-pilot (T/P) power ratio for transmission on the R-PDCH channel by a specific mobile station. Here, it should be appreciated that a maximum traffic-to-pilot power ratio is needed for each subchannel of the R-PDCH channel.
Meanwhile, in controlling a reverse link data rate in a mobile communication system supporting hybrid automatic repeat request (HARQ) transmission, for example, a system as provided in IS-2000 Revision D, a transmission delay time is generated when a mobiles station responds to ACK/NACK information from the base station. That is, in the HARQ system, error control is achieved through a retransmission of a data packet in response to a negative acknowledgement (NACK) signal provided on a forward link, i.e., via a forward acknowledgement channel (F-ACKCH). If an acknowledgement (ACK) signal is received, the mobile station transmits a new packet. Thus, the data traffic on the R-PDCH channel is made up of new packets and retransmission packets.
Referring to FIG. 3, illustrating a reverse link data rate control method in a HARQ system, the aforementioned transmission delay can be divided into a plurality K of subchannels for providing homogeneous and heterogeneous services, and each subchannel can be established with an independently set data rate. In the example of FIG. 3, K=4. During call establishment, the base station uses signaling to inform a mobile station of the number of subchannels, e.g., one, two, three, or four, that can be used for packet transmission by the mobile station. Accordingly, if a mobile station occupies four R-PDCH subchannels for transmitting traffic data via a series of four subchannel transmissions, a base station should transmit four grant messages, as shown in FIG. 4, to separately control the data rate of each sub-channel.
Therefore, in the reverse link data rate control method of the related art, the transmission of multiple grant messages, equaling the number of subchannels assigned to a mobile station, is required. Such transmission is required even if transmitting the same information in each message. Hence, the base station must repetitiously transmit data, which consumes transmission power unnecessarily.